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Old 01-25-2008, 04:11 PM   #61
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Eric---

Wrongo. An outboard does not produce a rolling effect on a boat. The engine would have to be horizontal to the boat's axis to do that. Torque acts around the axis of the rotating parts of an engine. On an outboard, the rotating parts are vertical, not horizontal. So when this twisting force is applied to the transom, it will yaw the boat, not roll it. Take the cover off a multi-cylinder outboard and look at how the spark plugs are arranged. They're in a line going straight up ( or two lines if it's a V-engine). The pistons are horizontal but pistons do not produce torque, the rotating power of the crankshaft does. And the crankshaft is vertical (as is the driveline). So the effect on the boat is as if you were above it, reached straight down and grabbed the transom in the middle, and rotated your hand. The boat movement this produces is yaw not roll.

This should be a no brainer to visualize. When you rev the engine of a car, the car leans (rolls) to the side away from the direction of the crankshaft rotation. The engine in a car is mounted horizontally which is why the direction of roll is ninety degrees to the ground. Now take that car and hang it vertically on a rope tied to the front bumper and rev the engine. The car will rotate the same direction as it did before, but since it's vertical, the direction of roll will be parallel to the ground. If the ground was a boat, and the back of the vertical car was fastened to the back of the boat, the torque would try to pivot the boat not roll it. This is yaw.

If I open the throttle on our 17' Arima fishing boat which is powered by a 90 hp, three-cylinder Yamaha, the boat turns a bit to one side. This is the result of the torque from the engine trying to twist the transom. It is not the result of anything the propeller is doing, because the slide (yaw) is actually in the opposite direction than prop walk would be if there was enough prop walk to make any difference (there isn't).

And you need to learn to read a bit better. I was not comparing a boat with two 240 hp engines to a boat with one 120 hp engine, nor was I comparing a boat with one 240 hp engine to a boat with two 120 hp engines. I was comparing a boat with two 120 hp engines to a boat with one 120 hp engine. Same model boat in each case, same model engine.

Anyway, your basic premise is totally wrong. I don't know how the hell you came up with the notion that a single 240 hp engine will have the same maintenance costs as a boat with two 120 hp engines. The very fact you have two engines doubles many of the maintentnace costs right off the bat. You have to buy twice as many filters, twice as many belts, twice as much (or nearly twice as much) oil, service twice as many injectors, replace twice as many heat exchangers, plus the labor costs to service the engines will be double because it takes twice as much time to service two engines as it does one.



-- Edited by Marin at 17:17, 2008-01-25
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Old 01-25-2008, 04:14 PM   #62
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Marin, you are assuming a rear wheel drive longitudinally mounted engine in the car in your example.....which isn't the norm these days. I'mm just poking fun at ya....(and tryig to keep up with ya post wise)!!!)
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Old 01-25-2008, 04:21 PM   #63
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John---

You're right. The torque from a transverse engine will cause the nose or tail of the car to squat depending on the rotational direction of the engine.

PS-* You fly a 767 right?* Well, steer clear of Paris.* A tug driver there just shoved a 767 backwards into a blast fence and destroyed the entire 48 section (tail cone) up to the aft bulkhead.* We're sending an AOG team to fix it.* So if you go to Charles de Gaulle, check out the credentials of the tug driver before you let him shove your airplane around.......

-- Edited by Marin at 17:35, 2008-01-25
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Old 01-25-2008, 07:05 PM   #64
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Marin, luckily RWD is coming back into style. There aint nothin' like American Muscle causing a car to twist on it's chassis.

And no, we don't do CDG in the 767 out of Houston(we might out of EWR). It is 777 out of Houston to CDG.* It does amaze me*how that we hire people for 10 bucks an hour and expect them to*handle a $150,000,000 airplane.*

If it ain't Boeing, I ain't going!!!!!

-- Edited by Baker at 20:07, 2008-01-25
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Old 01-26-2008, 03:49 PM   #65
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RE: Electric Boat Engines

Marin

I have no further words for you. You just don't get it.

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Old 01-26-2008, 05:05 PM   #66
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Your riight. I can't get what doesn't make sense
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Old 01-26-2008, 08:26 PM   #67
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RE: Electric Boat Engines

I think Marin is talking about the torque produced by the rotation of the engine crankshaft and his posts make perfect sense.

I think Eric might be talking about the torque produced by the rotation of the prop.

I think....
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Old 01-26-2008, 09:01 PM   #68
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Gene---

You are correct. I have been talking about the torque from the engine, motor, powerplant, whatever one wants to call it, of an outboard. I assumed that's what Eric had been talking about since he used the term "engine" all the time.

But if he's been talking about the torque from the propeller, which given its size on an outboard isn't going to be all that much, then yes, he's correct, the prop torque will act around the longitudinal axis of the boat and, if there's enough of it, will roll or lean the boat in the direction opposite the rotation of the propeller.

I would be surprised if the small prop on an outboard produces enough torque to induce a noticeable roll or lean to one side--- I've never noticed it in our 17' Arima. But most outboard boats are small enough that the distribution of the weight in the boat, including people and in our case a dog walking around would cause the boat to be out of lateral trim one way or the other most of time anyway so any torque-induced lean from the propeller would be impossible to discern.
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Old 01-27-2008, 05:07 AM   #69
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My little 16ft bow rider has a noticable lean to*port when on a plane. I have always assumed that it was prop torque that caused it and wished for a set of trim tabs. The reason I thought prop torque was because I have shifted weight around the boat while on plane moving people, swaping drivers etc with no appreciable change in the*port lean. If not prop torque what is it?

-- Edited by Jim Spence at 07:31, 2008-01-27
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Old 01-27-2008, 11:38 AM   #70
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One way to help determine that is to compare the direction of lean to the rotation of the prop. If the boat leans to port then the prop needs to turn clockwise as viewed from the rear of the boat if propeller torque is what is causing the lean.
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Old 01-27-2008, 01:12 PM   #71
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Of course I was talking about propeller torque as we were talking about propeller walking. SPENCE i'm sure the roll input in your dingy is propeller torque and yes a small propeller can cause a significant torque imput as it's turning quite fast. The propeller does turn 3000 rpm and frequently we run them at full throttle. Also outboard propellers typically have a lot of pitch and that ads a lot to the tendency to walk. Tecnically Marin was right about outboard engine torque but it's totally fly stuf and off the subject. Of course we've been off the subject for at least a week. I'm mad at Marin but I do feel a bit sorry for him reguarding his whining about the high costs of twin engine maintanance....he needs to buy 30 quarts of oil for an oil change! I guess a lot of other guys on this forum are in the same boat (pi). Speaking of the lot of you I'll bet 90% of you have hydraulic steering and as a result almost none of us will be the recipants of propeller walking as felt at the helm. There is almost no feedback to the helm with HS and quite a bit with cables or gears. This reminds me of the near need to install HS with the auto pilot when I take the plunge. Also I think most all twin screw boats have counter rotating propellers and anyone can count for himself the next time in a sizeable boat yard by looking at the many exposed propellers.

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Old 01-29-2008, 11:17 AM   #72
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Eric---

Once again, failure to read correctly. I'm not whining about the "high cost of twin engine maintenance." If I didn't want that cost, we wouldn't have bought a twin. Not rocket science here, buddy. I simply stated that the maintenance costs of two engines is basically double that of single engine. What you get for your money is easier maneuverability--- not better maneuverability, just easier--- and you don't have to come home on the end of a rope if you have a problem that requires an engine to be shut down.

And another incorrect statement from you is with regards to the need to hold a correction with cable-chain steering because of "feedback."** We used to charter a single-engine GB with cable-chain steering and there was no, repeat NO, need to hold any rudder against your theoretical prop walk because-- surprise, surprise--- there isn't any prop walk at cruise speeds. *If I let go of the wheel the boat continued to track straight ahead until current, waves, or wind started moving it off course.* In fact that single engine GB36 tracked better than our twin GB36 because of the single's slightly longer keel and larger rudder.

Another incorrect assumption on your part is the notion that you need, or should have, hydraulic steering if you want an autopilot. All Grand Banks boats up to fairly recently have cable-chain steering, and a HUGE number of them have autopilots. Our boat had an autopilot until we removed it.

You did say one thing right.* Almost all--- in fact today I would venture to say virtually all-- twin-engine boats have counter-rotating props.


-- Edited by Marin at 12:24, 2008-01-29
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Old 01-29-2008, 11:23 AM   #73
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Cable/chain steering has MUCH better feel than hydraulic...hydraulic has no feel at all.
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Old 01-29-2008, 01:31 PM   #74
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Another advantage to using cable/chain steering, and a major one as far as I'm concerned, is that one can "center" the wheel (helm) and KNOW that the rudder is centered.* I brought a 50 footer back to New Orleans from Miami a number of years ago using hydraulic steering and the lack of knowing what the rudder was doing and the total lack of feel (feedback) was discouraging at best.

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Old 01-29-2008, 02:01 PM   #75
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BUT, maybe what Eric was getting at is that an Autopilot has a much easier time with hydraulic steering than it does with chain/cable....almost all "natural loads" are muted. So you could really put an undersized autopilot on a boat with hydraulic steering.
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Old 01-29-2008, 02:19 PM   #76
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John--

I think it depends on how the autopilot drives the steering system. On our boat, and most GBs that I am familiar with, the autopilot actually turns the wheel. So the steering effort the autopilot has to overcome is no different than what you experience turning the wheel. On our boat, for example, you can turn the wheel with one finger--- there is almost no resistance at all in the cable/chain system. The Benmar autopilot our boat had when we bought it had the control unit in the engine room under the helm console with a short 1:1 ratio "bicycle chain" drive up to a sprocket on the axle of the wheel at the helm station. So the autopilot drive motor felt exactly what we feel when we turn the wheel--- virtually no resistance.

But.... some newer autopilots use a ram in the lazarrette to drive one of the rudder bars (or the only rudder bar if it's a single-engine boat). On a cable/chain system, this requires a hell of a lot more force because you are driving the entire steering system backwards. You can experience something of what this is like by grabbing an outboard motor on the back of a boat with consol steering and turning it from side to side as opposed to turning the wheel. On our 17' Arima, for example, I can turn the wheel with a finger. But I have to put my whole weight behind turning the motor itself against the resistance of the rack and pinion steering system. Same thing on our GB--- when I need to move the rudder bars to one side when I'm working in the lazarette, it takes a lot of arm power to do this. So among the GB gurus, anyway, a hydaulic lazarette rudder bar ram-type of autopilot is not recommended because of the strain it puts on the cables and sheaves of the steering system.

My experience with hydraulic steering and autopilots is very old--- from the 1970s with a Uniflite in Hawaii---- but I seem to recall that on some of them when the autopilot is engaged the hydraulic steering is somehow disengaged. If it wasn't, the autopilot would not be able to drive the rudder bar back and forth at all. But I'll admit I may be misremembering this.
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Old 01-29-2008, 06:41 PM   #77
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RE: Electric Boat Engines

Marin,
Will you please quit responding to my posts.
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Old 01-29-2008, 07:27 PM   #78
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I'll be delighted to as long as you quit claiming I said things I didn't say.
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Old 01-29-2008, 11:09 PM   #79
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First off, y'all behave yourselves!

ANd Marin, on a hydraulic boat with A/P, the steering is still available with the A/P engaged. It relly depends on how fast the pump is and how fast it can correct the inputs you make on the wheel. It won't hurt it because there is no direct connection....only fluid. But some boats have very fast pumps and your input will be corrected fairly quikly.

Also, Marin, what I was referring to ref "natural loads", was more related to sailboats. In a sailboat you get "weather helm" due to the balance/imbalance of the sail trim. An A/P has to continuously correct for this and hydraulics make it easier on the A/P.
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Old 01-30-2008, 12:55 AM   #80
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John---

Thanks for the clarification of the relationship between and autopilot and hydraulic steering. I don't remember where I heard the bit about needing to uncouple the hydraulic steering in order to use the hydraulic autopilot. Either I misunderstood, was wrong, or what was being talked about was some sort of home-made autopilot system.

No argument regarding the need to hold pressure on the helm or tiller of a sailboat. That's apparent even with the tiller of the little Montgomery 7-11 sailing dinghy we carry on our aft cabin.

Most of the autopilots I've seen on sailboats have been called 'self-steerers" by their owners. I know what they look like but I've never had any of our friends who have them actually explain how they function. But they apparently have the ability to control the main rudder via the tiller or main steering system to keep the boat in the same relationship with the wind that the sails are set for.

Or the main rudder is locked in position and the self-steerer has a small rudder that makes the corrections needed to keep the boat in a constant relationship to the wind's direction.* I guess I'm just going to have to get one of our friends to explain the thing to me now

But if I understand its function properly, it's not so much an autopilot that holds a constant compass heading but a device that keeps the boat at a constant angle to the wind. Because if the boat was on a constant compass heading and the wind shifted a bit, the sails would go out of trim. I knew guys in Hawaii who would set the "self-steerer" to keep the boat trimmed to the wind and then they'd go below to sleep for a few hours. As I understand it, the boat would not hold a constant compass heading or follow a course, but since on the open ocean the wind would keep blowing in more or less the same direction, the boat would keep going more or less where the skipper wanted to go. Holding to an exact course was no so important out there, as you could make corrections when you woke up.

But I could be wrong on my understanding of all this......

-- Edited by Marin at 01:57, 2008-01-30
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